Recently developed diesel engines are provided with a turbo intercooler intake system for decreasing an amount of exhaust emissions. Such diesel engines are generally called turbo diesel engines.
As is generally known, a turbo diesel engine includes a turbocharger for pressurizing intake air using energy of exhaust gas, and an intercooler for cooling the pressurized intake air. In such a turbo diesel engine, pressurized air is supplied to combustion chambers so that the amount of oxygen supplied to the combustion chambers is increased. Therefore, combustion characteristics can be improved, and fuel economy can also be improved because pumping loss that is generated when introducing air into a cylinder can be decreased.
In such a turbo diesel engine, fuel injection is controlled mainly based on boost pressure, which means the pressure within the air intake pipe downstream of the turbocharger. If the driver depresses the acceleration pedal while the engine is operating, an engine control unit calculates a required amount of fuel in proportion to the amount of acceleration pedal travel (i.e., an acceleration pedal value).
If the calculated required amount of fuel is less than a predetermined smoke fuel limit that is determined according to an intake air pressure, fuel injection control is performed by a calculated required amount of fuel. However, if the calculated required amount of fuel is greater than the predetermined smoke fuel limit, fuel injection control is performed by a predetermined smoke fuel limit. Here, the smoke fuel limit is a maximum amount of fuel that is determined according to pressures of intake air such that incomplete combustion due to a shortage of an intake air can be prevented.
Under such fuel injection control, even though a required amount of fuel is increased by the acceleration pedal value, an amount of fuel corresponding to the predetermined smoke fuel limit is injected if a current pressure of intake air is not sufficiently high.
Fuel injection timing is maintained as a value that is determined based on engine speed (rpm) regardless of the intake air pressure. If the acceleration pedal is abruptly depressed for rapid acceleration, a required amount of fuel frequently becomes greater than the smoke fuel limit. In this condition, because the actual amount of fuel is limited to the smoke fuel limit, an increase of energy of exhaust gas due to an increase of an amount of injected fuel is delayed. Consequently, rotational speed of the turbocharger is also delayed, so that a time lag for the intake air pressure to reach a value corresponding to the required amount of fuel occurs. A delay of an increase of the intake air pressure brings about a delay of an increase of engine torque. This phenomenon is generally referred to as a turbo lag.
In a TCI (Transistor Controlled Ignition) diesel engine, the effect of turbo lag on power efficiency is significant, and an engine having a relatively large turbo lag has poor acceleration performance and consumes much more fuel during acceleration. If the smoke fuel limit is determined as a relatively high value in order to improved acceleration performance, the amount of smoke due to incomplete combustion increases substantially.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.